Field
This application relates to balance and equalization of rechargeable electrical energy cells constructed into a battery of a plurality of series connected cells.
A system and method for the charge control and cell charge redistribution for a plurality of series connected rechargeable electrical storage cells where the objective of equalization of individual cell is based on the optimization criteria of remaining coulombmetric capacity.
Prior Art
Batteries consisting of series connected cells of varying energy delivery capabilities create a challenging problem for cell charge management of both energy replenishment and depletion. In the discussion that follows a system for the optimization of charge storage and delivery for a battery, composed of a series of cells, based on individual cell charge and redistribution management is disclosed.
Historically rechargeable batteries, consisting of series and parallel connected electro-chemical cells, have been charged through a port common to both charge and discharge as shown in FIG. 1. This arrangement is simple, efficient and effective for many battery technologies and applications. Battery chemistries such as Lead-Acid and Ni—Cd are tolerant of over charge, the condition where cells are charged beyond the full charge state, and over discharge, the condition where cells are depleted below a recoverable threshold. Though robust, these technologies are at a disadvantage regarding both gravimetric and volumetric energy densities to Lithium based cells. Li-Ion cells in particular are intolerant of over and under charge. In the case of over charge, any cell voltage within the battery must be prohibited from exceeding a prescribed voltage limit in order to avoid explosion and/or fire while over discharge below a prescribed voltage need be avoided less a permanent loss of energy capacity will ensue. Therefore, per the configuration of FIG. 1, a Li-Ion battery consisting of a series connected cells is full when any of the constituent cells (1,2) is full, at which point charging (4) must cease. Alternatively, the battery is empty when any of the cells (1,2) is empty, at which point the load (3) must be removed to avoid damage to the battery.
The discharge characteristics of individual Li-Ion cells are shown in FIG. 2. The cells within a battery become imbalanced in remaining coulombmetric capacity to empty (abbreviated as RCCE) due to a number of factors including manufacturing variations and thermal gradients within the battery. The cell 5 of FIG. 2 has a coulombmetric capacity of 80% of that of cell 6.
In this illustration the full to empty energy storage capacity of cells 5 and 6 are 2.96 and 3.7 W Hrs respectively (6.66 W Hrs total). When these two cells are combined in series within a battery, both in a full charge state, the energy available is 6.00 W Hrs with a 0.66 W Hrs residual in cell 6 unavailable to the load due to cell 5 reaching the over discharge state. Specifically the current that flows from one cell is the same that which flows through the other (where charge is the integration of current over time). Alternatively if both cells were charged from mutual depletion to the point where cell 5 is full their combined energy available would be 5.85 W Hrs.
A simplified static load model for a cell is shown in FIG. 3. The cell (7) may be modeled by a voltage source (9) which represents the Open Circuit Voltage (abbreviated as OCV) in series with the internal impedance R (8). R (8) varies as much as 15% within the same manufacturing lot and is dependent on cell temperature, state of charge (coulombmetric % full, abbreviated as SOC) and time at rest. Variations in R are on the order of 50-100% as a function of SOC in the SOC range of 100-15%. FIG. 4 shows the impact on terminal voltage of the internal impedance as a function of load where the same cell is discharged at a rate of rated capacity (abbreviated as C) divided by 40 in (10) while curve (11) is discharged at a rate of C.
Traditionally charging of a depleted cell is a two step process, shown in FIG. 5, beginning with a constant current phase (12) followed by a constant voltage phase (13). A rapid charge from empty to the constant current/voltage transition (14), latter referred to as the ‘Knee’, yields 65 to 75% of the cells coulombmetric capacity in as little 40 minutes while the remaining constant voltage charge to full will typically take twice or more this amount of time to complete. Full charge determination is specified by the manufacturer to achieve rated capacity but is typically chosen by the system designer to be the condition where the charge current falls below a threshold, such as C/30, following the charge Knee (14). Note the power consumed in the charge process, denoted by Relative Power on the graph, drops significantly at constant/voltage transition (14). As a reference the last 30% of remaining coulombmetric capacity to full (abbreviated as RCCF) during charge equates to approximately 32% of the total energy storage for the cell while the last 30% in discharge to empty represents 27%.
It should be noted that the properties of charge and discharge prohibition limits apply to various other rechargeable electrical energy storage, including super-capacitors, and therefore all descriptions are equally valid and equivalent with regards to these technologies.
Prior art describes systems to equalize cell voltages and SOCs. Given the variation in electrical resistance as a function of SOC and the intrinsic difference in coulombmetric capacity of series connected cells, equal voltage or SOC of the constituent cells does not equate to equality in remaining coulombmetric capacity except at depletion or full. In these systems battery charge must be continuously moved from cell to cell to achieve voltage or SOC equality. Also note that SOC is a relative measurement of capacity where a specific reduction in SOC of one cell may not equate to an equal reduction of SOC in another series connected cell. Referring to FIG. 2, if cell 5 and 6 are paired and equalized at 3.7 volts, half capacity point of cell 5, there exists a 12% coulombmetric capacity deficit between the two cells which need be closed by battery empty. In a constant power high-discharge application, such as an electric vehicle or power tool, the voltage cell balancer competes with the load for the maximum discharge current which is problematic in that as the battery voltage declines the current demand increases. The advantage of the described system and method is that once RCCE equality during discharge or RCCF equality during charge is achieved no additional redistribution is required to achieve the stated goals.
A note on practice and terminology; the system or systems described subsequently lend well to the modular design of battery packs. In this concept a block can be composed of series and parallel connection of cells configured and managed by the system in the manner described. The battery can be composed of series and parallel blocks configured and managed by the system described, and so on, until aggregated down a load and MAINS connection. Any reference using the term cell in this document has the same meaning as a reference to a plurality of parallel connected cells. RCCE, measured in Amp Hours or coulombs, is the amount of charge that would be extracted from a cell if discharged to the empty voltage threshold from the present level. RCCF, measured in Amp Hours or coulombs, is the amount of charge that would be required to bring a cell to a full charge state from the present charge level. The word charge, as a noun, is quantified in coulombs (the cell charge). Charge, as a verb, is the application of current quantified in Amps (charge the cell).